Nuclear Quadrupole Resonance (NQR) testing is used for detecting the presence or disposition of specific substances, such as polycrystalline substances. It depends on the energy levels of quadrupolar nuclei, which have a spin quantum number I greater than xc2xd, of which 14N is an example (I=1). 14N nuclei are present in a wide range of substances, including animal tissue, bone, food stuffs, explosives and drugs.
In NQR testing, a sample is placed within or near to a probe comprising a radio-frequency (rf) coil and is typically irradiated with pulses or sequences of pulses of electromagnetic radiation having a frequency which is at or very close to one or more resonance frequency of the quadrupolar nuclei in a substance which is to be detected. If the substance is present, the irradiant energy will generate a precessing magnetization which can induce voltage signals in a coil adjacent the sample at the resonance frequency and which can hence be detected as a free induction decay (f.i.d.) during a decay period after each pulse or as an echo after two or more pulses.
The invention preferably relates to the testing of what are hereinafter termed xe2x80x9cremotexe2x80x9d samples. Although the following definition is not exclusive, remote samples are typically samples which lie outside the plane of the excitation/detection probe, often at a distance away from the probe which may be comparable with or greater than the largest cross-sectional dimension of the probe. With remote testing, it is often only possible to access the sample from one side, for instance if the sample is buried or concealed. Testing of remote samples which can only be accessed from one side is often termed xe2x80x9cone-sidedxe2x80x9d testing; in such tests only the field from one side of the probe is utilised, the probe usually being shielded on the other side. Such a probe is referred to herein as an xe2x80x9copen endedxe2x80x9d probe.
More particularly, the invention relates to the detection of the presence of remote or other samples containing quadrupolar nuclei.
As an example, the present invention has particular application to the detection of 14N quadrupole signals in drugs, such as cocaine and heroin, concealed on or within the person, possibly using a hand-held probe. As another example, the invention may find application in the detection of buried or concealed explosives, for instance in airport security monitoring. Again, in industrial processes, it can be used to detect signals from quadrupole-containing materials. Such materials might be proteins in foodstuffs, or quadrupole containing substances on conveyor belts, inside furnaces or nuclear reactors or in chemically or physically hazardous surroundings in which the probe must be located away from the remainder of the testing apparatus, possibly even on one side only of the system. The probe may be located inside the pressure vessel of the nuclear reactor, which may be at extremes of temperature and pressure.
Research carried out pursuant to the present invention has shown that conventional open ended probes have the disadvantage of being susceptible to interference. One type of interference is due to external sources producing rf spikes at random points in time. This causes the probe to produce bursts of rf signals at the frequency to which it is tuned. Large amplitude bursts may seriously corrupt the NQR signal since this may be less than the preamplifier noise base. Signal averaging may help to remove the effect of rf bursts, but this will increase the time taken for testing.
Another type of interference comes from more stable sources of rf energy at a single frequency, such as amplitude modulation (am) or frequency modulation (fm) radio transmissions. This type of signal may produce a line that could be confused with, or obscure, the NQR response.
Furthermore, in practical situations such as the detection of buried explosives or airport security monitoring, spurious interference may be generated by objects or matter surrounding or in the vicinity of the substance to be detected. Examples of such spurious interference are the piezo-electric signal generated in quartz, dry sand or soil by the electric field of the rf pulse, or the signal generated by ferromagnetic objects in response to the rf pulse. The spurious interference may be large enough to obscure or obliterate the NQR signal and to saturate the rf preamplifier and subsequent amplifiers.
The present invention aims to provide suppression of interference from whatever source. The interference may be any unwanted signal such as noise, and may originate from an external interfering source, or from the sample, as is the case with spurious interference, or from the NQR apparatus itself.
According to one aspect of the present invention there is provided an apparatus for Nuclear Quadrupole Resonance testing a sample in the presence of interference comprising excitation applying means for applying excitation, a first antenna for detecting a response to the excitation together with the interference, and a second antenna for detecting the interference, the first and second antennas being arranged such that a signal commonly detected in the two antennas is attenuated relative to a signal that is detected by only one of the antennas.
The response signal will normally be detected mainly in one antenna, while the interference will normally be detected in both antennas, and hence the apparatus according to the invention can afford the advantage that interference may be attenuated or cancelled.
In order to ensure that large spurious interference, which could potentially overload the testing apparatus, does not reach the antennas, the apparatus preferably comprises a screen located adjacent the first antenna, the screen being arranged to attenuate the electric component of an rf signal passing through it relative to the magnetic field component. For example, the screen may comprise one or more layers of a non-ferromagnetic metal such as aluminium or copper, or one or more layers of metallised plastic.
In order to prevent eddy currents from forming in the screen, which may cause the Q of the antenna to drop, the screen may be slotted. Preferably, the screen has a slot extending from its centre to its edge, although other slotting arrangements may be used.
The screen may be any suitable shape; in one preferred example the antenna is annular with a slot from the centre to the edge.
The apparatus preferably comprises a second screen, similar to the first screen, located adjacent the second antenna. This may help to ensure that the Q""s of the two antennas are as closely matched as possible.
In order to reduce further any spurious interference, the excitation applying means is preferably adapted to apply phase cycled pulse sequences, preferably according to the doctrine of phase equivalence as taught in U.S. Pat. No. 6,208,136 in the name of British Technology Group Limited, the subject matter of which is incorporated herein by reference.
Hence the apparatus may be for Nuclear Quadrupole Resonance testing a sample containing quadrupolar nuclei and which may give rise to spurious signals which interfere with response signals from the quadrupolar nuclei, in which case the excitation applying means may be adapted to apply a pulse sequence to the sample to excite nuclear quadrupole resonance, the pulse sequence comprising at least one pair of pulses, and the apparatus may further comprise means for comparing, for the or each such pair, the respective response signals following the two member pulses of the pair, the pulse sequence being such that the respective spurious signals following the two member pulses can be at least partially cancelled by the comparing means without the true quadrupole resonance signals being completely cancelled, and for the or each such pair, the two member pulses being of like phase. The sample may comprise a substance containing quadrupolar nuclei which may itself give rise to spurious signals, or the sample may comprise a first type substance containing quadrupolar nuclei and a second type substance which may give rise to the spurious signals.
For the or each such pair of pulses, a respective pulse preceding each member pulse of the pair may be of differing phase. The or each such pair of pulses may be of a first type, and the pulse sequence may further comprise at least one further second type pair of pulses, corresponding to the or each first type pair, but having cycled phases.
The excitation applying means may comprise an additional antenna for applying the excitation. However, preferably at least the first antenna is adapted to apply excitation. This can reduce the size and complexity of the apparatus.
The second antenna may also be active, that is, it may be adapted to apply additional excitation and to detect a response to that excitation. This may provide the advantage of allowing twice the area to be tested at any one time than would be possible if only the first antenna were active. Having both antennas active may also provide the advantage of reducing spurious interference, particularly in the case where the object or matter causing the spurious interference is in the field of view of both antennas.
In one preferred embodiment, the first antenna comprises a first coil and the second antenna comprises a second coil, the first and second coils being wound in the opposite sense to each other. The two coils may be connected in series or in parallel.
In another preferred embodiment, the first antenna is coupled to the second antenna via a balanced bridge circuit. This may allow the relative orientation and distance apart of the two antennas to be adjusted, and may also allow the two antennas to be more accurately balanced than in the case where the two antennas are directly connected.
In a further preferred embodiment the first antenna is coupled to the second antenna by a comparator circuit, for example an operational amplifier, or any other type of comparator.
Means may be provided for adjusting the relative orientation of the first and second antennas, to allow the common mode rejection of the two antennas to be optimized.
According to a closely related aspect of the invention there is provided a method of Nuclear Quadrupole Resonance testing a sample in the presence of interference comprising applying excitation to the sample, detecting a response to the excitation together with the interference to yield a first signal comprising a response component and an interference component, detecting the interference to yield a second signal, and combining the first signal and the second signal such that the interference component is attenuated relative to the response component.
The method may further comprise applying additional excitation and detecting a response to said additional excitation together with the interference to yield said second signal.
The interference may comprise spurious interference from ferromagnetic or piezoelectric materials.
The method may be a method of Nuclear Quadrupole Resonance testing a sample containing quadrupolar nuclei and which may give rise to spurious signals which interfere with response signals from the quadrupolar nuclei, in which case the step of applying an excitation signal may comprise applying a pulse sequence to the sample to excite nuclear quadrupole resonance, the pulse sequence comprising at least one pair of pulses, and the method may further comprise comparing, for the or each such pair, the respective response signals following the two member pulses of the pair, the pulse sequence being such that the respective spurious signals following the two member pulses can be at least partially cancelled by the comparison without the true quadrupole resonance signals being completely cancelled, and for the or each such pair, the two member pulses being of like phase. The sample may comprise a substance containing quadrupolar nuclei which may itself give rise to spurious signals, or the sample may comprise a first type substance containing quadrupolar nuclei and a second type substance which may give rise to the spurious signals
For the or each such pair of pulses, a respective pulse preceding each member pulse of the pair may be of differing phase. The or each such pair of pulses may be of a first type, and the pulse sequence may further comprise at least one further second type pair of pulses, corresponding to the or each first type pair, but having cycled phases.
The second signal may be detected in a different environment from the first signal. The method may be carried out in the absence of an applied magnetic field.